The present device relates to a roller heating processor for subjecting a flexible sheet such as paper or film to heat processing such as thermal development and thermal fixation. More particularly, the present invention relates to a thermal development apparatus for subjecting a photosensitive material to thermal development by putting the material into contact with the peripheral surface of a heating drum.
A roller heating processor for subjecting a sheet to heat processing such as thermal development and thermal fixation typically includes a heating roller for heating the sheet, and a backup roller for pushing the sheet against the heating roller. The sheet is transported between backup roller and the heating roller, such that the sheet can be moved while in pressure contact with the heating roller so that the sheet undergoes heat processing. The surface of the backup roller is made of an elastic material, e.g., silicone rubber, so that the sheet can be securely pushed against the heating roller by the backup roller. Consequently, the backup roller is also sometimes referred to as a soft roller. Typically, such heating rollers are made of an aluminum tube to obtain high thermal conductivity of the roller, and a coating layer of Teflon or the like on the surface of the tube. The heating roller is also sometimes referred to as a hard roller. A tubular heating lamp, e.g., a halogen lamp, is provided in the heating roller so that the lamp extends in the axial direction of the roller.
The roller heating processor is required to heat the sheet at a uniform temperature for a prescribed period of time. In other words, it is necessary that the distribution of the temperature of the heating roller in the axial direction thereof be uniform, and the sheet kept in pressure contact with the heating roller for the prescribed period of time.
It is known that latent images formed on various photosensitive materials can be made visible by thermal development. For example, in a conventional thermal development apparatus, a photosensitive material is wound on the peripheral surface of a heating drum which is rotated, and the material is kept in contact with the drum for a prescribed time while being conveyed, so that the material is heated to undergo thermal development.
If the heating time is relatively short as for thermal fixation in a plain paper copying machine, a laser printer or the like, typically, one need only heat the sheet between both rollers. However, if the heating time is relatively long as in thermal development of a thermosensitive diazo paper, the rotational speed of the rollers needs to be relatively low. As a result, a problem arises in that the heating of the sheet is more time-consuming.
To solve the problem, it has been proposed to provide a plurality of backup rollers along a heating roller, and a guide is provided a short distance from the peripheral surface of the heating roller between the backup rollers, as disclosed in the Japanese Unexamined Published Application No. 172046/89. Since the guide is located a short distance from the peripheral surface of the heating roller, it is not likely that the guide will cause the sheet to be heated by the heating roller between backup rollers. Although the sheet is more likely to be heated by the heating roller between backup rollers when the guide is located in contact or pressure contact with the heating roller, another problem results. Namely, since the heating roller has a coating layer of Teflon or the like which provides a lower coefficient of friction to make it more likely that the sheet put in pressure contact with the heating roller by the backup rollers slips on the heating roller, it often occurs that the sheet does not move together with the peripheral surface of the rotating heating roller between the backup rollers but becomes caught on the fixed guide, which typically has a coefficient of friction higher than that of the heating roller. As a result, there is a problem in that the sheet slackens, causing it to wrinkle in the leading edge portion.
Additional problems result when the sheet is a film for making a second original. If the sheet is paper or the like, the guide does not cause a problem, but serves to accurately heat the sheet to subject it to development. However, if the sheet is a film for making a second original, the guide often causes a problem, namely, the leading edge portion of the sheet is not properly heated to undergo perfect development, and an imperfectly developed streak-like area extending from the leading edge portion of the sheet to the center of the surface thereof is sometimes formed. The cause of this problem is presumed to be as follows. Since the guide does not contact the heating roller between the backup rollers, the leading edge of the sheet is perfectly heated on each of the backup rollers but not sufficiently heated between the backup rollers to undergo perfect development. Further, the remaining portion of the sheet comes into tight contact with the heating roller due to the two or more backup rollers so as to be properly heated to undergo perfect development.
Also, when the sheet is a film for making a second original, there is a problem in that a spot, a projection and/or indentation can be formed on the sheet, and/or an imperfectly developed streak-like area arises on the sheet. As a result of various studies on the cause of that problem, it has been determined that the problem has a subtle relationship with the quality of the film and the conveyance speed of the backup rollers, which is described as follows. When plural backup rollers are provided, sprockets on the rollers can be made different from each other in diameter or number of teeth so as to rotate the downstream backup roller slightly faster than the upstream backup roller to pull the sheet to prevent it from slackening and wrinkling as well as to keep the sheet in tighter contact with the heating roller.
A further problem exists when the sheet is a film for making a second original. If the sheet is a film, the photosensitive material applied thereon absorbs a relatively large amount of water when in a highly humid atmosphere so that when the sheet is put in between the most upstream backup roller and the heating roller and heated by the heating roller in pressure contact therewith, the water is quickly evaporated. When the sheet is thereafter pulled by the downstream backup roller and put in tight contact with the heating roller so that the sheet is heated to undergo development, the water vapor confined in and/or on the sheet because of the imperviousness of the sheet causes the abovementioned spot, projection, indentation and/or imperfectly developed streak-like area. If the sheet is paper or the like, it is so pervious that water vapor goes out of the sheet through the reverse side thereof so as not to cause the above-mentioned problem.
The problems due to absorption of water by the photosensitive material are described more generally below. When the carrying body of the photosensitive material is made of moisture-absorbing substance such as paper, the material absorbs a certain amount of water from the ambient air or absorbs a large amount of water in a highly humid atmosphere. When a sheet of the photosensitive material having the water is heated by the heating roller of a conventional thermal development apparatus, the water is evaporated. However, the water vapor thus generated on the roller contact side of the sheet is confined between the peripheral surface of the heating roller and the side of the sheet which is in contact with the peripheral surface of the heating roller. If the water vapor remains confined between the sheet and the peripheral surface of the heating roller, the photosensitive material of the sheet will not sufficiently undergo thermal development, so that the density of the visible image on the sheet is made partially lower, which makes the visible image nonuniform. In other words, the water vapor confined between the sheet and the peripheral surface of the heating roller prevents the photosensitive material from being uniformly heated for thermal development. As a result, the densities of different portions of the visible image on the photosensitive material differ from each other. This undesirable nonuniformity becomes more conspicuous as the sheet is more rapidly conveyed.
A thermal development apparatus disclosed in the Japanese Unexamined Published Application No. 104648/84 has been proposed to attempt to overcome the absorptivity problem. In the apparatus therein disclosed, a hole or a slit is provided in a heat transfer member so that water vapor generated on the obverse and reverse sides of the photosensitive material can be discharged through the hole or the slit. However, since the heat transfer member has the hole or the slit, it is complicated to make and difficult to manufacture. Further, since the apparatus is not used alone but provided in an imaging machine or the like and used as a part thereof, the water vapor discharged through the hole or the slit stays in the machine to make the humidity therein abnormally high if the water vapor is not discharged from the machine. The water vapor in the machine is, however, itself a problem because the high humidity is undesirable for any integrated circuits provided in the machine, such as those controlling the conveyance of the photosensitive material and the density of the recorded images.